Different from many organs, the heart previously was felt to be a terminally differentiated, postmitotic organ. This led to the assumption that the heart contained a fixed number of cells postnatally, and that if injury led to myocyte death, the myocardium would need to maintain its functional role with a reduced number of cells. While striated muscle cells of cardiac origin undergo terminal differentiation shortly after birth in mammals, it has been shown that myocardial regeneration occurs following injury. This concept led to an interest in the potential of stem/progenitor cells in the heart. Endogenous cardiac progenitor cells (CPCs) are capable of limited myocardial regeneration at sites of injury, however the therapeutic potential of exogenously administered progenitor cells has been an intense area of investigation, as heart failure due to myocardial injury remains a difficult and deadly problem for millions of Americans. Thus, a major objective of this application is to further understand pathways responsible for the fate of CPCs (both during development and postnatally into adulthood), and to understand the ability of these progenitor cells to divide and differentiation into functional cardiomyocytes. Myosin light chain kinases (MLCK) are a family of proteins that are important for myocyte function. One member of this family is the striated preferentially expressed gene (Speg). The Speg locus generates four different gene isoforms, with Speg1 and Speg2 being expressed preferentially in striated muscle (including muscle of cardiac origin). Speg1 and Speg2 share homology with MLCK family members, and along with obscurin-MLCK, are unique members of the MLCK family as they contain two tandemly arranged serine/threonine kinase (MLCK) domains. Prior investigations in our laboratory revealed that Speg isoforms (particularly Speg1) are markers of striated muscle differentiation. However, the functional significance of Speg isoforms was yet to be elucidated. We disrupted the Speg gene locus in mice, and revealed that homozygous mutant (Speg-/-) hearts began to enlarge by 16.5 days post-coitum (dpc), and by 18.5 dpc showed a marked dilation of right and left atria and ventricles. These dilated Speg mutant hearts demonstrated poor function, and a phenotype consistent with a dilated cardiomyopathy. Speg-/- mice also experienced significant neonatal mortality. Interestingly, the hearts of Speg-/- mice showed a reduced number of cells per mm3 of tissue compared with Speg+/+ mice, and a less differentiated appearance by transmission electron microscopy (EM), suggesting an alteration in the development of cardiac parenchymal cells. The overall goal of the application is to elucidate the role of Speg in the fate and differentiation of CPCs into functional cardiomyocytes, and to ascertain the importance of Speg in cardiac injury, repair, and function. To achieve this goal, we propose the following Aims: 1) investigate the role of Speg in CPC fate, commitment to the cardiomyocyte lineage, and cardiomyocyte differentiation;2) decipher the mechanisms responsible for the development of cardiac dysfunction in the absence of Speg;&3) determine the importance of Speg during cardiac injury (pressure overload) in adult mice. PUBLIC HEALTH RELEVANCE: Cardiovascular diseases have been the leading cause of death in the United States for more than 80 years, recently accounting for 35.3% of all deaths in the United States. Heart failure is a very prevalent disease process that contributes to deaths associated with cardiovascular diseases, and the incidence of heart failure remains extremely high with 670,000 new cases diagnosed in the United States in 2006. This application will provide new insight into the differentiation and maturation of cardiac muscle cells, and determine the importance of this differentiation process on overall heart function in newborn and adult hearts that have been injured.